Controlled p-Type Doping of Pyrite FeS2

Pyrite FeS2 has extraordinary potential asa low-cost,nontoxic, sustainable photovoltaic but has underperformed dramaticallyin prior solar cells. The latter devices focus on heterojunction designs,which are now understood to suffer from problems associated with FeS2 surfaces. Simpler homojunction cells thus become appealingbut have not been fabricated due to the historical inability to understandand control doping in pyrite. While recent advances have put S-vacancyand Co-based n-doping of FeS2 on a firmfooting, unequivocal evidence for bulk p-doping remainselusive. Here, we demonstrate the first unambiguous and controlled p-type transport in FeS2 single crystals dopedwith phosphorus (P) during chemical vapor transport growth. P dopingis found to be possible up to at least similar to 100 ppm, inducing similar to 10(18) holes/cm(3) at 300 K, while leaving the crystalstructure and quality unchanged. As the P doping is increased in crystalsnatively n-doped with S vacancies, the majority carriertype inverts from n to p near similar to 25and similar to 55 ppm P, as detected by Seebeck and Hall effects, respectively.Detailed temperature- and P-doping-dependent transport measurementsestablish that the P acceptor level is 175 +/- 10 meV above thevalence band maximum, explain details of the carrier inversion, elucidatethe relative mobility of electrons and holes, reveal mid-gap defectlevels, and unambiguously establish that the inversion to p-type occurs in the bulk and is not an artifact of hoppingconduction. Such controlled bulk p-doping opens thedoor to pyrite p-n homojunctions, unveilingnew opportunities for solar cells based on this extraordinary semiconductor.